The present invention relates to a process for producing a ceramic body for surgical implantation that has high strength and toughness, as well as good compatibility with bones.
A conventional process that is gaining an increasing interest in the art of producing ceramic bodies for surgical implantation consists of coating a high-strength ceramic material with another ceramic layer having good compatibility with bones. Among typical substrate materials are dental ceramics, polycrystalline alumina, and single-crystal alumina. Apatite is commonly used as the coating material. Japanese Unexamined Published Patent Application No. 118411/1978 entitled (in translation) "Apatite Coating Ceramics and Process for Producing the Same" discloses a method wherein an apatite powder is applied to the surface of a sintered substrate made of one of the materials listed above, followed by sintering the assembly.
Partially stabilized zirconia sintered body has recently drawn the interest of researchers because of its high strength and toughness. However, if this sintered body is coated with an apatite powder and sintered at 1,300.degree. C., which is the usual sintering temperature for apatite, the resulting apatite coat adheres so weakly to the substrate that the former is easily rubbed off with finger pressure. Satisfactorily strong adhesion is not obtained even if the sintering temperature is increased to approximately 1,500.degree. C. On the other hand, if the sintering temperature is elevated to about 1,600.degree. C., the strength of the substrate made of partially stabilized zirconia sintered body is significantly reduced.
Most of the partially stabilized zirconia ceramics have sintering temperatures between 1,500.degree. and 1,600.degree. C., which are considerably higher than the sintering temperature of apatite (1,300.degree. C.). Due to this factor, strong adhesion is unobtainable between a zirconia ceramic substrate and apatite layer. Additionally, if zirconia ceramics are subjected to another heating cycle at elevated temperatures, excessive growth of crystal grains coupled with a change in volume due to phase transformation induces cracking, which leads to a significant drop in the strength of the final product. Because of these unique characteristics of the partially stabilized zirconia ceramics as compared with the other conventional dental ceramics and alumina ceramics, it has been difficult to achieve strong adhesion by coating apatite powder to the zirconia ceramics surfaces.